1. Field of the Invention
The present invention relates to an electroless plating solution, particularly to a stabilized solution which forms no nodules and which is capable of forming a plating film having a smooth surface.
2. Description of the Related Art
In forming a plating film on a substrate, it is possible to use an electroless plating method which deposits metal thereon by chemical reduction.
An electroless plating method is a method which, in general, deposits a metal plating film continuously in an electroless plating solution containing a metal salt and a reducing agent by a chemical reaction taking place among these substances. The electroless plating solution is either acidic or alkaline. For example, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 4-116175, there is an electroless plating solution containing nickel(II) sulfate, hypophosphorous acid as a reducing agent and citric acid as a complexing agent.
The great advantages of an electroless plating method are that a conductive metal coating can be formed on a suitably pretreated nonconductor, and that a plating film having a uniform thickness can be formed on a substrate having any shape. Accordingly, electroless plating is a technology which has heretofore been widely utilized for decoration, rust prevention, magnetic discs, pattern formation on a printed board, and the like.
Specifically, the electroless plating method has been used, for example, for the ground plating of a wiring pattern in the method for producing printed boards by the additive procedure.
That is, as shown in FIG. 11, a via hole 90 is provided in a substrate 91. A plating resist 92 is then formed in a portion of the surface of the substrate 91 where a pattern is not formed.
Electroless plating is subsequently conducted to form ground plating 931 in pattern-formed portions and in the interior of the via hole, and then electroplating is conducted to form thick copper plating 932. A wiring pattern 93 consisting of the ground plating 931 and the thick copper plating 932 and via hole plating 930 are thus formed.
An electroless copper-nickel plating as a ground plating for forming a printed board enhances the bonding strength between the substrate 91 and the thick copper plating 932 as disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 6-318771.
However, since a metal salt and a reducing agent are always in a reactive state in the electroless plating solution, the electroless plating solution is thermodynamically unstable. Ensuring the stability of the electroless plating solution is, therefore, the most important problem.
Accordingly, various stabilizers containing metal ions such as, for example, lead ions have heretofore been proposed. Further, Japanese Unexamined Patent Publication (Kokai) No. 4-116175 proposes a stabilizer containing an ammonium salt, and Japanese Unexamined Patent Publication (Kokai) No. 4-157169 proposes a stabilizer containing a ferrocyanide and a sulfide.
However, the use of lead is not preferred from the standpoint of environmental conservation. Moreover, it cannot be concluded that any of the prior technologies has fully achieved stabilization of the electroless plating solution.
Furthermore, an unstabilized electroless plating solution causes various problems during the formation of the wiring pattern. That is, since the wiring pattern 93 is required to be made fine, the widths of the pattern and the gaps among patterns have been made narrow. Accordingly, extraordinary depositions in the narrow gaps among lines must be avoided. The extraordinary depositions are termed nodules, and cause short circuits. The applicants believe that the nodules are formed by the decomposition reaction of the electroless plating solution which takes place when the concentration of oxygen playing the role of a stabilizer in the solution almost reaches zero near the plating reaction sites.
When nodules are formed in the ground plating 931 and are then coated with the thick copper plating 932, the copper plating also follows the nodules to become likely to cause short circuits between the lines.
However, the electroless copper-nickel plating method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 6-318771 sometimes produces nodules on the ground plating when a wiring pattern having a line width as narrow as up to 100 .mu.m is formed.
In view of the problems as mentioned above, the present invention is intended to provide an electroless plating solution capable of ensuring its stability and of preventing the formation of the nodules.